[{"_id":ObjectId("6143450cc0318c23d8f18424"),"id":"3","name":"prod03","children":[{"_id":"6143440ac0318c23d8f1841f","qty":10},{"_id":"614344b1c0318c23d8f18422","qty":100}],"totalQty":110},{"_id":ObjectId("614344b1c0318c23d8f18422"),"id":"2","name":"prod02","children":[],"totalQty":100},{"_id":ObjectId("6143440ac0318c23d8f1841f"),"id":"1","name":"prod01","children":[],"totalQty":10}]

db.collection.aggregate([{"$match":{"id":"3"}},{"$addFields":{"children":{"$map":{"input":"$children","as":"c","in":{"_id":{"$toObjectId":"$$c._id"},"qty":"$$c.qty"}}}}},{"$graphLookup":{"from":"collection","startWith":"$children._id","connectFromField":"children._id","connectToField":"_id","as":"children"}},{"$addFields":{"children":{"$map":{"input":"$children","as":"c","in":{"id":"$$c.id","name":"$$c.name","qty":"$$c.totalQty"}}}}}])